Electrochemical processes utilizing a layered membrane

ABSTRACT

The invention relates to a membrane assembly and processes for the utilization of the membrane assembly, or membrane assemblies having like properties. The membrane assembly comprises intermixed layers of capillary material and high dielectric constant impermeable material, forming capillary channels parallel to the direction of ion transport through the membrane. The membrane is anion permeable, does not have membrane potential and will not foul even over extended operation, allows cross-flow of anions and cations, and is highly efficient. It is possible to remove complex metals from any contaminated acid by electrodialysis, such as removing vanadium and uranium in recoverable form from contaminated phosphoric acid, while producing food grade phosphoric acid in the process. Additionally, simple metals may be removed from mine waste liquids (from leaching), chlorine can be produced from a chloride containing salt, and chromium can be removed from chromium contaminated water by electrolysis. Milling sludge can be treated to form water, caustic, and acid, and mineralized water can be treated to form de-mineralized water, by subjecting the feed liquids to electrodialysis.

This is a division of application Ser. No. 957,876, filed Nov. 6, 1978,now U.S. Pat. No. 4,242,193.

BACKGROUND SUMMARY OF THE INVENTION

The invention relates to a membrane assembly for electrochemicalprocesses, and to processes that may be practiced utilizing the membraneassembly of the invention, or a membrane assembly having likeproperties. An important feature of the membrane assembly of the presentinvention is that it allows the passage of anions therethrough, whileeffectively retarding the passage of water therethrough. The membraneassembly according to the present invention is capable of continuouslong-term operation. The term "extended operation", as used in thepresent specification and claims is intended to mean successfuloperation over at least hundreds of hours, if not months and years.

In the past there have been numerous proposals for anion permeablemembranes for use in electrochemical cells, such as "amberlite" resins,formed by bonding together resin particles, beads or granules. Suchmembranes, and like ion exchange membranes, are not capable of extendedoperation, fouling quickly, and require the application of such a largeamount of energy as to be totally impractical, can easily be clogged,can react chemically with ions transmitted thereby, are highlysusceptible to radiation damage, pass hydration water, are subject toswelling and/or are permselective.

According to the membrane assembly of the present invention, however, itis possible to provide a membrane which avoids the disadvantages ofprior art anion-selective membranes and additionally is superior in manyrespects to conventional cation-permeable membranes. The membraneassembly, according to the present invention, is anion-permeable; itwill not readily pass water; allows cation and anion cross-flow (is notpermselective); is inexpensive, simple to make, and uses inexpensive andeasy-to-make materials without any special processing; is generally notdegraded by radioactivity, providing generally excellent radioactive ionseparation; does not swell as a result of chemical reactions therein; isnot susceptible to clogging or ready degradation; does not have amembrane potential (is free of anions and cations), therefore does noteffect an increase in the water head on one side of the membrane (withassociated overflow problems), except for very weak electrolytes; hasincreased electrical efficiency (high Faraday current efficiency withlow voltage drop); and can be constructed of a variety of materials andconfigurations, therefore the exact material and design parameters canbe chosen to exactly fit a particular situation. Additionally, utilizedin special forms, all back-diffusion is prevented, and when utilized inparticular cell configurations, has a mass-transfer efficiency of wellover 100% (e.g. 170%), caused by the combination of electrolyticalcurrent-efficiency and electro-pherotic effects.

The membrane assembly according to the present invention comprises aplurality of layers of substantially void-free capillary material, eachlayer being thin enough so that it retards the passage of watertherethrough, and thick enough so that it allows passage of ionstherethrough, the capillary material having a positive angle of wetting;and means for forming the capillary material into directed capillarychannels allowing passage of anions and cations therethrough, and meansfor preventing cross-flow of water and ions between the capillarychannels. The forming and preventing means preferably comprises aplurality of separation layers of inert and impermeable material, havinga high dielectric constant and smooth surfaces, and means formaintaining the capillary and separation layers in position so that theseparation layers are interposed with the layers of capillary material,all of the layers being substantially parallel, so that anion and cationtransport across the assembly through capillary material layers takesplace, but ion and water transport across the separation layers does nottake place. The capillary material may be chosen from a wide variety ofconventional capillary materials, including paper, asbestos, wood,synthetic felts, and polyester and polypropylene woven or non-wovenwebs, and normally will be chosen so that it has a thickness of about0.001 to 0.03 inches. In order to decrease the voltage drop, while stillproviding proper functioning, the capillary material normally has adimension in the direction of ion transport of about 0.1 to 1.5 inches.The separation layers may be comprised of a wide variety of conventionalinert and impermeable material, having high dielectric constant, such asnon-conducting plastic films (preferably polyethylene film), rigidplastic plates, ceramic and glass.

In order to prevent essentially all back-diffusion, and for otherpurposes, a membrane assembly according to the present invention may beprovided in combination with a second said assembly and a film membraneassembly, such as described in my patent application Ser. No. 814,715,filed July 11, 1977, now U.S. Pat. No. 4,124,458, the disclosure ofwhich is hereby incorporated by reference in the present application.The film membrane assembly comprises a film of substantiallywater-impermeable, ion-impermeable, insulating material having athickness of about 0.001 to 1 mm. That combination includes means (e.g.glue) for maintaining the film membrane assembly sandwiched between thecapillary assemblies, with the capillary assemblies in intimate contactwith the film and with the film being disposed substantiallyperpendicular to the direction of ion transport across the capillaryassemblies. The capillary membrane assemblies disposed on either side ofthe film membrane comprise means for providing a continuation of theboundary layer thereof and for eliminating membrane potential and, thus,fouling of the membrane.

According to the present invention, electrochemical cells are providedcomprising an anode, disposed in an anode chamber; a cathode, disposedin a cathode chamber; and a membrane assembly, disposed between theanode and the cathode, the membrane assembly comprising an anionpermeable, semi-permeable or water impermeable, non-permselectivemembrane, capable of providing passage of anions thereacross overextended operation without destructive swelling, clogging, chemicalreaction, or consumption thereof. The various chambers and membranes canhave a wide variety of configurations. For instance, the membranes maybe planar, curvilinear in the dimension perpendicular to ion transport(i.e. cylindrical or tubular), curvilinear in the direction of iontransport (i.e. waviform), and may be provided as a large membrane withthe anode and cathode chambers defined by cutouts in the membrane (theareas of the membrane defining the anode and cathode chambers beingtreated so that they are effective to prevent diffusion of materialsfrom the anode and cathode chambers thereinto).

An exemplary cell according to the present invention may comprise adialysis cell, including at least two membranes disposed between theanode and the cathode, and defining at least one central chambertherebetween, at least the membrane adjacent the anode chambercomprising a said anion permeable membrane. In order to lower theresistivity of the cell, and for acting as an electrode, and for actingas a barrier to water and metal transport when no current is applied tothe anode, a plurality of graphite particles may be disposed in at leastone central chamber. Three central chambers preferably are provideddefined by at least three of said anion permeable membranes, each of thecentral chambers including a plurality of graphite particles disposedtherein. In utilizing such a dialysis chamber, it is possible to obtainmass transfer efficiencies of over 100% (e.g. 170%)!

According to another method of the present invention, it is possible toform uranium and vanadium rich liquid from conventional phosphoric acidcontaminated with the same. Conventional phosphate rock and overburdencontains from 50 to 200 ppm of uranium, and the uranium is also found inthe phosphoric acid produced from the ore in the production offertilizers, and the like, and is found in the gypsum pond water fromthe processing plant. According to the present invention, the phosphoricacid at any concentration below 100% is selected from some convenientpoint in the process (i.e. after filtration or just before evaporation)and is subjected to electrodialysis. In addition to producing uraniumand/or vanadium rich liquids according to the present invention,aluminum, iron, magnesium, calcium, and the like are removed--resultingin a better grade of metals and fluorine depleted--phosphoric acid--andgypsum pond water, which heretofore has been a significant disposalproblem is even harmlessly disposed of. In fact, even food-gradephosphoric acid can be produced. According to the method of uraniumand/or vanadium removal of the present invention, an electrochemicalcell is utilized comprising an anode chamber defined by an anionpermeable membrane, a cathode chamber defined by a cathode permeablemembrane, and at least one central chamber between the anode and cathodechambers. Water is originally provided in the anode chamber and gypsumpond water in the cathode chamber. Contaminated phosphoric acid is fedinto the central chamber, a current is supplied to the anode sufficientto effect electrodialysis of the contaminated phosphoric acid, theuranium and/or vanadium rich supernatant liquid is withdrawn from thecathode chamber, and the metal-depleted phosphoric acid is withdrawnfrom the anode chamber. Where a plurality of central chambers areprovided, each having a plurality of graphite particles disposed thereinand defined by anion permeable membranes, food-grade phosphoric acid maybe withdrawn from the anode chamber.

According to another method of the present invention, it is possible toeffectively and efficiently, over extended operation, demineralize waterby subjecting mineral-containing water to electrodialysis. Themineral-containing water may be hard water, sea water, salt water, orbrackish water. Again, a dialysis cell as described above is utilized,and the method is practiced by providing water as the anolyte andcatholyte, feeding the mineral-containing water into the bottom of thecentral chamber. Metal-containing liquids and metal precipitates arealso produced in the cathode chamber, while halogens and acids areproduced in the anode chamber.

According to a still further method of the present invention, it ispossible to remove chrome from chrome-contaminated rinse water fromchromium plating operations, etching liquid, or cooling tower effluent.It is possible to effect chrome removal to such an extent that the waterproduced has less than 0.05 pp. chromium, and the chromium isconcentrated to a sufficient extent so that it is recoverable and/orreusable (e.g. chromic acid having a concentration of about 15% or morecan be produced). The method is practiced utilizing an electrolysis cellwhich may comprise a plurality of series-connected cathode chambersbeing separated by an anion permeable membrane, and the method beingpracticed by feeding chromium-contaminated water to the bottom of thefirst cathode chamber, supplying current sufficient to effectelectrolysis to the anode, withdrawing water having less than 0.01 ppm(i.e. 0.05 ppm) chromium from the top of the last cathode chamber, andwithdrawing concentrated chromic acid from the anode chamber.

Further processes practiced according to the present invention includethe treatment of simple metal in a recoverable form, by electrolysis,production of chlorine and/or metal from a chlorine-containing salt andwater, treating ECM sludge to produce nitric acid and sodium hydroxide,by electrodialysis, and treating phosphate slime (clay and phosphaterock mixed together) to produce phosphoric acid.

It is the primary object of the present invention to provide an improvedmembrane assembly, and processes that are capable of efficiently andeffectively, over extended operation, purifying phosphoric acid whileproducing uranium and/or vanadium rich liquids, demineralizing water,removing chromium from chromium-contaminated water, turning ECM sludgeand other milling wastes into reusable components, producing chlorine,and producing phosphoric acid from phosphate slime. This and otherobjects of the invention will become clear from an inspection of thedetailed description of the invention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagramatic exploded side view of an exemplary membraneassembly according to the present invention;

FIGS. 2 and 3 are modifications of the membrane assembly of FIG. 1illustrated in use in electrochemical cells;

FIG. 4 is a diagramatic view illustrating the formation of an exemplarymembrane assembly according to the present invention, that iscurvilinear in the dimension perpendicular to the direction of iontransport;

FIG. 5a is a top plan view of an exemplary electrochemical cellarrangement formed by a machined membrane according to the presentinvention;

FIG. 5b is a side view of another embodiment of an exemplary membraneassembly according to the present invention utilizing the film membraneassembly between two capillary membrane assemblies;

FIGS. 6a, 6b and 6c are schematic illustrations of the purification ofphosphoric acid that may be practiced according to the presentinvention;

FIG. 7 is a side view of an exemplary arrangement for demineralizingwater according to the present invention;

FIG. 8 is a side view of an exemplary electrodialysis cell for purifyingECM sludge according to the present invention;

FIG. 9 is a side diagramatic view of an exemplary electrolysis cell forpurifying simple metal mine waste;

FIG. 10 is a perspective diagramatic view of an exemplaryelectrochemical cell for removing chromium from chromium-contaminatedwater; and

FIGS. 11a and 11b are electrolysis cells for chlorine production fromNaCl and CaCl₂ respectively.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary membrane assembly according to the present invention isillustrated generally at FIG. 10 in the drawings. The membrane assembly10 comprises a plurality of layers 11 of substantially void-freecapillary material, such as paper, asbestos, wood, synthetic felts andpolyester and polyproplene woven or non-woven webs, a wide variety ofother conventional capillary materials also being utilizable. Thecapillary material chosen has a positive angle of wetting--that is,water disposed in a capillary tube formed of the material will have ameniscus with peripheral wall-engaging portions elevated above thecenter of the water column. Each layer is thin enough so that it allowsthe passage of ions therethrough; preferably the capillary materiallayers each have a thickness of about 0.0001 to 0.030 inches (about0.003 to 0.01 inches being preferred); and means 12 for forming thecapillary material into directed capillary channels, allowing passage ofanions and cations therethrough, and for preventing cross-flow of waterand ions between the capillary channels. The means 12 preferablycomprises a plurality of separation layers 13 of inert impermeablematerial, having a high dielectric constant and smooth surfaces (seeFIG. 1 in particular), and means 14 for maintaining the capillary andseparation layers 11, 13 in position so that separation layers 13 areinterposed with layers 11, all of the layers 11, 13 being substantiallyparallel, so that anion and cation transport across said assembly 10through said capillary material 11 takes place (in dimension A), but ionand water transport across separation layers 13 (in dimension B) doesnot take place. In order to minimize voltage drop across the assembly10, it is formed so that the layers 11 have a dimension in the directionof ion transport (A) of about 0.1 to 1.5 inches (0.25 to 0.5 inchesbeing preferred). Suitable materials for forming the separation layers13 include non-conducting plastic films (e.g. polyethylene film 0.001inches thick), rigid plastic plates (see 15 in FIG. 2), ceramic, andglass.

The maintaining means 14 may take a wide variety of forms, includingthose illustrated in FIGS. 1 and 3. In FIG. 1, means 14--which are shownin dotted line--comprise top and bottom plates 16 formed of inert,substantially rigid material, the plates 16 applying a compressive forceon layers 11, 13 when the nuts 17 cooperating with bolts 18 passingthrough the plates 16 are tightened. The pressure applied by the plates16 need not be large, but merely sufficient to provide a uniformmembrane assembly, which is tight enough to minimize undesirableelectrolyte diffusion and/or leakage across or through the membrane. Themaintaining means may instead comprise, as illustrated schematically at19 in FIG. 3, a resin or like structure for maintaining the layers 11,13 together in unpressurized form, completely surrounding same. Themembrane assembly is then inserted in electrolyte, where a moderateamount of swelling takes place due to the absorption of water by thecapillary material layers, the internal pressure resulting beingsufficient to prevent any undesired leakage or the like. No excessiveswelling takes place since there are no chemical reactions within themembrane itself. Also, as illustrated in FIG. 3, perforatednon-conducting (e.g. plastic) plates 20 may be provided on either sideof the layers 11, 13 to support the membrane assembly.

The layers 11, 13 may be disposed so that a single layer 11 is providedbetween two layers 13, or multiple layers 11 (e.g. 3 or 5) may beprovided between adjacent layers 13. Additionally, the maintaining means14 may maintain layers in a curvilinear configuration in a dimensionperpendicular to the direction A of ion transport, or even in acurvilinear configureation in the dimension A ion transport, withoutadversely affecting the operation of the membrane assembly. FIG. 4 is aschematic showing of the formation of a tubular or cylindrical membraneassembly 10', produced by winding a plurality of layers 11 of capillarymaterial from rollers 21 with a layer 13 of separation material fromroller 22.

FIGS. 5a and 5b are further illustrations of other exemplary forms amembrane assembly 10 can take according to the present invention. InFIG. 5a, the element 24 represents a plurality of large sheets ofcapillary material 11 and separation layers 13 stacked one on top of theother, and held together by bolts of the like passing through openings25 therein, cutouts 26 being milled or otherwise formed, out of themember 24 to provide a series of membrane assemblies 10 spaced from eachother, the cutouts 26 having electrolyte disposed therein and formingthe anode and cathode chambers, and central chambers defined by membraneassemblies 10 between the anode and cathode chambers. In order toprevent diffusion of materials from the anode and cathode chambers intothe material 24, the entire area of the member 24 aside from theportions 10 themselves is treated with a material effective to preventdiffusion into the member 24.

The membrane assembly illustrated in FIG. 5b is a sandwich membraneassembly utilizing the capillary membrane assemblies 10 according to thepresent invention as "forcers" for a film membrane such as shown inapplication Ser. No. 814,715. The capillary membrane assemblies disposedon either side of the film membrane comprise means for providing acontinuation of the boundary layer thereof and for eliminating membranepotential and, thus, fouling of the membrane. The film membrane 28, asshown in application Ser. No. 814,715 comprises a film of substantiallywater-impermeable, ion-impermeable insulating material having athickness of about 0.001 to 1 mm; polyethylene is a particularlysuitable material. The assemblies 10 are disposed on one or both sidesof the film 28 in intimate contact therewith, and means are provided formaintaining the film membrane 28 sandwiched between the capillarymembrane assemblies 10, with the film 28 being disposed substantiallyperpendicular to the direction A of ion transport across the capillarymembrane assemblies 10 forming a sandwich 29. The maintaining means maytake any suitable form, such as glue around the periphery between thecooperating faces.

Exemplary electrochemical cells according to the present invention areillustrated schematically in FIGS. 2, 3 and 6a through 11d. Eachelectrochemical cell includes at least an anode 30 disposed in an anodechamber 31, a cathode 32 disposed in a cathode chamber 33, and amembrane assembly 10 disposed between the anode 30 and the cathode 32.In practice of the processes according to the present invention,generally the membrane assembly utilized can comprise the capillarymembrane assembly 10 illustrated in FIGS. 1 through 4, a sandwichmembrane 29 as illustrated in FIG. 5b, or a film membrane assembly asdescribed in application Ser. No. 814,715. Under most circumstances, thecapillary membrane assembly 10 is preferred since it suffers less damagefrom neutrons and alpha particles in the cell membrane, and since it isless expensive and easier to make. However, under some circumstances thefilm membrane or the sandwich membrane 29 will be preferred, thesandwich membrane 29 being especially useful in situations where it isnecessary to prevent all backdiffusion, the film membrane and thesandwich membrane 29 being hydraulically impermeable.

In situations where an electrodialysis cell is used (.e. FIGS. 6, 7 and8) at least the membrane 40 defining the anode chamber 31 will be ananion permeable membrane according to the present invention (capillarymembrane assembly 10, sandwich membrane assembly 29, or film membrane asillustrated in Ser. No. 814,715). While it is preferred that themembrane 41 defining the cathode chamber 33 also comprise a membraneaccording to the invention, other cation permeable conventionalmembranes may also be utilized, although they may foul quickly. Themembrane assembly according to the present invention comprises an anionpermeable, non-ionic, semi-permeable or water impermeable,non-permselective membrane capable of providing the passage of anionsthereacross over extended operation without destructive swelling,clogging, chemical reaction, or consumption thereof.

According to the present invention, the method of forming functionalanionic liquid and cationic material from a source liquid or semi-solidcontaining ions with a Faraday efficiency of at least 90% is provided,consisting essentially of the step of effecting electrodialysis orelectrolysis of the source liquid. Under some circumstances, the masstransfer efficiency is greater than about 150% (e.g. 170%). Where thesource liquid contains acid and complex metals, the functional cationicmaterial produced includes metal hydroxide precipitates and asupernatant liquid (e.g. pure water, uranium and/or vanadium solutions),and the functional anionic liquid produced includes metal-depleted acid.One practical method practiced according to the present invention isillustrated schematically in FIGS. 6a through 6c. This method effectsthe formation of uranium and/or vanadium rich liquid from phosphoricacid contaminated with same. Phosphoric acid produced during themanufacture of fertilizer or the like from phosphate rock generally iscontaminated with significant amounts of uranium, vanadium, and metals,such as aluminum, iron, calcium and magnesium. Such metals areundesirable in the phosphoric acid, and if the metals are not removed,can result in an unacceptable end product. Additionally, thecontaminants--especially uranium and vanadium--have significantcommercial value themselves. Further, in the utilization of phosphoricacid, gypsum pond water containing CaSO₄ H₂ O is produced which is asignificant disposal problem. According to the present invention, bysubjecting the contaminated phosphoric acid to electrodialysis, it ispossible to produce uranium and/or vanadium rich liquids, metal-depletedor even food grade phosphoric acid, metal precipitates, and fluorine,while at the same time providing disposal of gypsum pond water.

One exemplary way of practicing the phosphoric acid treatment accordingto the present invention is illustrated in FIG. 6a, an electrodialysiscell being provided with a membrane 40 defining an anode chamber 31, andpreferably with the membrane 41 also being a non-ionic, semi-permeableor water-permeable, non-permselective membrane as according to thepresent invention, capable of providing passage of ions there acrossover extended operation without destructive swelling, clogging, orchemical reaction or consumption thereof. Water may be provided as theoriginal electrolyte in the anode chamber 31, and gypsum pond water maybe provided as the electrolyte (providing a weak-acidic electrolyte) inthe cathode chamber 33, and the contaminated phosphoric acid is fed intothe central chamber 42. A current is supplied to the anodes sufficientto effect electrodialysis of the contaminated phosphoric acid, with themetals passing through the membrane 41 into the catholyte, andmetal-depleted phosphoric acid--both from the gypsum pond water and fromthe contaminated acid in the central chamber 42--passing through themembrane 40. At the completion of the electrodialysis, the catholytecomprises a volume of metal precipitates (e.g. aluminum and ironprecipitates), with the supernatant liquid containing uranium andvanadium in solution. In situations where the original phosphoric aciddoes not contain uranium, or vanadium, the supernatant will be purewater, however, virtually all phosphoric acid from phosphate rockcontains at least some uranium. The final anolyte comprisesmetal-depleted phosphoric acid which is eminently suitable forfertilizer production or other uses. Once the final anolytes andcatholytes are formed, the uranium and vanadium can be separated out byconventional processes (e.g. by utilizing ammonium carbonate, whichproduces ammonium uranyl tricarbonate, and after calcination, producesU₃ O₈).

In the embodiment illustrated in FIG. 6b, the same process is carriedout only in this situation a common anode 30 is provided for a pair ofcathodes 32.

When it is desirable to produce food-grade acid from the contaminatedphosphoric acid, an arrangement such as illustrated in FIG. 6c isutilized. In the apparatus of FIG. 6c, a plurality of central chambers42, 42', are provided. Each of the central chambers 42' is defined byanion permeable membranes 40 according to the invention, the chamber 42adjacent the cathode chamber 33 being defined on one side thereof bymembrane 41. Each of the central chambers has a plurality of graphiteparticles disposed therein. The graphite particles lower the resistivityof the entire electrochemical cell, act as an electrode, and act as abarrier to water and metal transport when no current is supplied to theanode. The contaminated phosphoric acid is fed into chamber 42, themetal precipitates and uranium and/or vanadium rich liquids are producedin the cathode chamber 33, and in the anode chamber 31 food-grade acidis produced. While any number of central chambers 42, 42' may beprovided it has been found that the three-chamber arrangement asillustrated in FIG. 6c is suitable for the production of food-gradeacids. Additionally, if desired, fluorine produced in the centralchambers 42, 42' may be collected (as by hood 44, shown diagramaticallyin FIG. 6c), which also is a commercial product.

According to the present invention a method of demineralizing water isalso provided, the method consisting essentially of the step ofsubjecting the mineral-containing water to electrodialysis. This notonly results in the production of pure water, but the minerals separatedfrom the water may also be utilized, depending upon their value andtheir original concentration. Thus, utilizing the invention, it ispossible to "mine" sea water, while producing fresh water therefrom. Themineralized water that is treated according to the present invention maybe sea water, salt water, brackish water, hard water or the like.

In practicing the water-demineralizing method according to the presentinvention, apparatus as illustrated in FIG. 7 may be utilized. Theapparatus is essentially identical to that illustrated in FIG. 6b,including a pair of "central" chambers 42, a pair of cathode chambers,and a common anode chamber. The mineralized water to be treated is fedinto the bottom of the central chambers 42, a current sufficient toeffect electrodialysis is supplied to the anode 30, and pure water iswithdrawn from the top of chambers 42. The metals pass through membranes41 into the cathode chambers, providing metal precipitates and/or metalsin solution as the catholyte, and the acids and halides pass throughmembranes 40 into the anolyte.

According to the present invention, it is also possible to treat ECMsludge (electrochemical milling sludge) to produce nitric acid andsodium hydroxide. This is accomplished by electrodialysis, asillustrated schematically in FIG. 8. A suitable electrodialysis cell asshown in FIG. 8 includes the anode and cathode chambers defined bymembranes 40, 41, and the central chamber 42. The ECM sludge is fed intothe bottom of the central chamber 42, and a current sufficient to effectelectrodialysis is supplied to the anode 30. The sodium ions in thesludge pass through the membrane 41 into the catholyte, forming sodiumhydroxide with water originally provided as the catholyte, and thenitric acid passes through the membrane 40 into the anolyte, wateroriginally being provided in the anode chamber 31. Pure water isproduced at the top of the central chamber 42, which may be withdrawn ifcontinuous processing is effected. Additionally, a small amount ofair-dryable non-toxic (metal hydroxides) sludge is produced, which formsat the bottom of the central chamber 42.

According to a further method of the present invention, it is possibleto treat simple metal waste liquids from mine leaching processes toobtain reusable leaching acid, water, and metal in a recoverable form.Such a method consists essentially of the step of subjecting the wasteliquid to electrolysis. Suitable apparatus for practicing this method isillustrated schematically in FIG. 9, including anode and cathodechambers 31, 32 and an anion permeable, non-ionic, semi-permeable orwater-impermeable, non-permselective membrane 40 according to theinvention, capable of providing passage of anions thereacross overextended operation without destructive swelling, clogging, chemicalreaction or consumption thereof. In practicing the method, the wasteliquid is fed into the bottom of the cathode chamber 33, with pure waterand metal precipitates (or perhaps metals in solution) being produced inchamber 33, and the anions passing through the membrane 40 to providemetal-depleted acid in the anode chamber 31.

The mine waste liquid recovery process according to the presentinvention is especially suitable for recovery of sulfuric acid andmanganese hydroxide from waste liquid from manganese mining. The wasteliquid comprises MnSO₄ in water, and sulfuric acid is produced in anodechamber 31, and pure water is produced as the supernatant liquid incathode chamber 33, with Mn(OH)₂ precipitate formed in a cathode chamber33. The sulfuric acid may then be reused for further leaching.

By practicing a further method according to the present invention, it ispossible to recover chromium-contaminated water, producing concentratedchromic acid and essentially pure water. This is accomplished bysubjecting the chromium-contaminated water to electrolysis, againpreferably utilizing the anion permeable, non-ionic, semi-permeable orwater-impermeable, non-permselective membrane according to the presentinvention, capable of providing passage of anions there across overextended operation without destructive swelling or clogging, chemicalreaction or consumption thereof. A preferred apparatus for practicingthis method is illustrated schematically in FIG. 10. In such anarrangement, the anode 30 is disposed at the center of the cylinder, themembrane 40 being provided as a tube concentric with the anode 30, thecathode 31 being tubular in form and being concentric with the membrane40 (the cathode preferably being formed to allow liquid passagetherethrough) and a casing 45 surrounds the entire assembly. Preferably,a plurality of baffles 46 are provided extending radially between thecasing 45 and the membrane 40 so that in effect a common anode chamber31 is provided cooperating with a plurality (e.g. six) cathode chambers33 formed there around, the baffles 46 being impermeable. Overflow means(cutouts and/or the dimensioning of the baffles 46) are provided betweenthe various cathode chambers 33 to provide for communication betweeneach of the cathode chambers 33. An inlet 47 is provided to one of thechambers 33, and an outlet 48 is provided for one of the chambers 33adjacent to the inlet chamber, overflow means being provided between allof the chambers except the chambers containing the inlet 47 and theoutlet 48. Chromium-contaminated water is fed into inlet 47 at thebottom of the inlet chamber 33, treated water is withdrawn from theoutlet 48 at the top of the last chamber, and concentrated chromic acidcollects in the anode chambers 31. Depending upon the end use to whichthe treated water is to be put (i.e. discharge, reused for chromiumplating rinse water, etc.) the process will be carried out to producewater of any purity desired. For discharge into the environment, thewater will be treated so that it has a concentration of less than 0.05ppm, and in fact, concentrations as low as 0.01 ppm have been obtained.Chromium is recovered from the concentrated chromic acid by conventionaltechniques, acid concentration of at least about 10-16% being possible.Again, the degree of acid concentration achieved will be dependent uponwhether or not, or to what extent, the chromium is to be recovered.

Depending upon the nature of the chromium contaminated water, it may bedesirable to oxidize the chrominum before feeding it to the inlet 47.For instance, where the contaminated water contains trivalent chromium,oxidation is practiced to oxidize the trivalent chromium to hexavalentchromium (which can be drawn into the anolyte). Additionally, whereorganics, cyanide, or cyanide metal complexes are present in thefeedwater, oxidation is practiced to effect oxidation of suchcomponents. A conventional oxidation unit is illustrated in block format 48 in FIG. 10 in the line feeding the contaminated water to the inlet47. The oxidizer 48 may be any suitable conventional or prior artoxidizer, such as shown in application Ser. No. 841,925 filed Oct. 13,1977 now U.S. Pat. No. 4,161,435.

Practicing a still further method according to the present invention, itis possible to produce chlorine by electrolyisis in a more efficientmanner than has heretofore been possible. In FIGS. 11a 11b, suitableelectrochemical cells for practicing the method of chlorine generationof the invention are illustrated diagrammatically, the cell in FIG. 11abeing used for the production of sodium hydroxide and chlorine fromwater containing NaCl while the cell of FIG. 11b generates chlorine fromwater containing CaCl₂.

In practicing this method, a chlorine containing salt and water are fedinto the anode chamber 31 and/or the cathode chamber 33, a currentsufficient to produce electrolysis is applied to the anode 30, and theCl₂ is withdrawn from the anode compartment 31, as indicateddiagrammatically in FIGS. 11a and 11b. Where the salt is NaCL and thesalt water is fed into anode compartment 31, the sodium ions passthrough the membrane 40 into the cathode compartment to form sodiumhydroxide, and are separately recoverable. When the NaCl and water isfed into the cathode chamber 33, the chlorine ions pass through themembrane 40 into the anode chamber 31, being liberated as Cl₂. In FIG.11b, calcium chloride is the salt, and when the salt and water are fedinto the anode compartment originally, the cations pass through themembrane 40 into the cathode chamber 33, forming Ca(OH), while thechlorine is liberated in the anode chamber 31. When the salt isoriginally fed into the cathode chamber 33, the chlorine ions passthrough the membrane 40 into anode compartment 31. By practicing themethod according to the invention, it is possible to produce chlorine(and sodium) directly from seawater with no pretreatment beingnecessary, and sodium and chlorine production can be achieved with muchhigher efficiency, especially in continuous operations. For instance, inconventional sodium cells, with ion exchange membranes NaOH is producedwith a concentration of about 28% at a transfer efficiency of about 60%,while according to the present invention NaOH with a concentration ofabout 40% can be produced with a transfer efficiency of about 80%.

While particular exemplary processes that may be practiced according tothe present invention have been set forth above, the membrane accordingto the present invention is also utilizable in and with many otherprocesses. For instance phosphoric acid can be produced from phosphateslime (clay and phosphate rock mixed together), which is one of thewaste products formed during the production of fertilizer from phosphaterock which presently cannot be properly disposed of. The membraneaccording to the invention can be used for the purification of any acidcontaining complex metals, in general producing anionic liquid andcationic material from a source liquid or semi-solid containing ions.This is usually accomplished with a Faraday efficiency of at least about90%.

In practicing the methods described above, and other methods with whichthe membrane according to the present invention is utilizable, the anodeand cathode materials will be selected from conventional materials thatare suitable for the particular process involved. All of the aboveprocesses are capable of extended operation without membrane foulingwhen utilizing membrane assemblies according to the present invention,being operable for at least hundreds of hours, if not for months andyears. There is no membrane potential of the membrane according to theinvention, and they essentially do not foul even during extendedoperation.

All of the processes according to the present invention may be practicedas batch processes or continuous processes, both of which are within thescope of the claimed invention.

EXAMPLE 1

Simulated rinse water from chrome plating rinsing having 209 ppmhexavalent chrome was continuously flowed at about 18-20 ml/min. througha system generally as indicated in FIG. 10. The cathode was graphiterods, the anode Pt plated titanium. The membrane was tubular, having anoutside diameter of 5" and an inside diameter of 3.75", and was 95/8"high. The membrane was formed by alternating three layers of capillarymaterial comprising Eaton-Dykman 950 paper, 0.007 in. thick and onelayer of separation material comprising conventional polyethylene foodwrapping 0.001 in. thick. The voltage applied was 34 volts, and theconcentration of chrome in the anolyte was 1565 ppm. During continuousoperation, the concentration of chrome achieved in the outlet water was5-10 ppm, this concentration is sufficiently low for recirculation ofthe rinse water.

EXAMPLE 2

Simulated chrome plating rinse water having 104 ppm hexavalent chromewas treated in a batch process utilizing the same membrane andelectrodes as set forth in EXAMPLE 1. 1.2 gals of rinse water wastreated with an applied voltage of 34 volts, and after about 19 hours ofoperation the concentration of chrome in the catolyte was about 0.58ppm. By increasing the voltage the concentration was reduced evenfurther, ultimately to less than 0.05 ppm. The same membrane as used inthis test and in EXAMPLE 1 was utilized continuously for hundreds ofhours without clogging, chemical degradation or swelling. Chromic acidhas been concentrated in the anolyte up to about 16%, and typicalFaraday efficiencies have been 15-25% at 100 ppm (feed water) and below,35-45% at about 1000 ppm, and 70-90% at 60,000 ppm and higher.

EXAMPLE 3

ECM Sludge, approximately 600 ml., was placed into the centercompartment of a dialysis cell as illustrated in FIG. 8. The anolyte andcatholyte were originally water. The cathode was 304 stainless steel andthe anode was expanded Pt sheet. A voltage of about 12 v was appliedover a period of about 4 hours. Both of the membranes were constructedof interlayers of 3 sheets of capillary material comprising Eaton-Dykman250 paper about 0.005 in. thick, and a width of about 0.375 in., and asingle layer of separation material comprising conventional polyethylenefood wrap 0.001 in. thick. At the completion of operation, the anolytewas approximately 600 ml. of 23% HNO₃, the catholyte was approximately14% NaOH, and the material in the central chamber comprised about 550ml. of H₂ O of a pH of about 5.8, and about 50 ml. sludge, which was inthe form of metal hydroxides approximately 90% of which was ironhydroxide. The sludge was non-toxic, and could be removed throughprecipitation, filtration, or evaporation.

EXAMPLE 4

Utilizing a system generally as illustrated in FIG. 6a, 500 ml. of rawcontaminated phosphoric acid from a fertilizer production process usingFlorida phosphate rock was fed into the middle compartment 42. Bothmembranes 40 and 41 were formed of 3 layers of Eaton Dykman 250 papereach about 0.006 in. thick to 1 layer of conventional polyethylene foodwrap 0.001 in. thick, the membranes being about 0.375 in. wide. Gypsumpond water (pH about 1) was added to the cathode chamber 33, and waterto the anode chamber 31. The cathode was graphite and the anode wasplatinized titanium. The initial voltage applied was about 4 volts, withan initial current of 10 amps. After 3-4 hours of operation, about 28%phosphoric acid was removed from chamber 31 having a metal content of1/2 the metal content of the raw phosphoric acid. Precipitates wereformed in the chamber 33, which when analyzed revealed aluminum, iron,and other hydroxides, and the supernatant liquid in chamber 33 was greenin color--indicating the presence of vanadium--and wasradioactive--indicating the presence of uranium. The concentration ofuranium in the liquid was determined to be about 140 ppm, and thevanadium concentration was estimated to be about 100 ppm.

EXAMPLE 5

Utilizing a system generally as illustrated in FIG. 6c, with thecompartments 42, 42' filled with Dixon #2 graphite flakes and all of themembranes 40, 41 as provided in Example 4, 500 ml. of raw phosphoricacid, as in Example 4, was introduced into chamber 42, with distilledwater in the other compartments. The initial voltage applied was 25volts, and this was reduced to 3-4 volts as the phosphate aniontransfered to the anolyte and steady state conditions were achieved, thecurrent being approximately 11 amps. at steady-state conditions. Afterapproximately 21 hours of operation, 25% food grade phosphoric acid wasproduced in the anolyte, having a specific gravity of about 1.24. Theacid transfer efficiency during testing was about 170% of which 80% areFaraday current efficiency and the rest produced by electrophoresis as aresult of the graphite being present.

EXAMPLE 6

Utilizing a system as illustrated in FIG. 11b, approximately 450 g. ofCaCl₂ was dissolved in 1 liter of water, and was placed in both theanode and cathode chambers, 31, 33. The membrane 40 was as described inExample 4, and further tests with substantially identical results wereconducted with a membrane 40 comprising alternating layers of asbestospaper about 0.002 in. thick and 0.375 in. in width, and conventionalpolyethylene food wrap 0.001 in. thick. The anode was platinizedtitanium, and the cathode was graphite. The applied voltage was held atabout 9-13 volts with a current density of approximately 0.27 amp/in.²(6 amp. with a membrane area of about 22 in.²). Chlorine was liberatedat the anode chamber 31 with an initial Faraday efficiency of about 60%which increased to approximately 89-95% after about 20 minutes whensteady state conditions were reached, staying at this level until thedepletion of the CaCL₂ became excessive. Utilizing the same potentialand current density with CaCl₂ in the anode chamber only, and utilizingan all-paper membrane, (i.e. not containing layers of polyethene foil)the maximum Faraday efficiency achieveable was 60-65%. Similar testswere done with the layered membrane described above with ferricchloride, magnesium chloride, and sodium chloride with similar voltageapplications, and similar Faraday efficiencies were achieved with each.

EXAMPLE 7

The demineralization of hard water was practiced utilizing apparatusgenerally as indicated in FIG. 7, only containing a single centralchamber 42 and one cathode chamber 33. 500 ml. of hard water having ahardness of 120 ppm (as CaCO₃) was introduced into central compartment42. The membranes 40, 41 were the same, each as described in Example 4.The cathode was 304 stainless steel and the anode was ruthenium dioxidecoated titanium. The voltage applied was about 30 volts, and the currentabout 1-2 amps. The current was applied for about 1 hr., after whichtime the water in chamber 42 was tested to have a hardness of 17 ppm (asCaCO₃).

EXAMPLE 8

Utilizing a system generally as illustrated in FIG. 9, 750 ml. ofmanganese-containing leach water was introduced into the cathodicchamber, with distilled water in the anode chamber. The chambers wereseparated with a membrane comprising alternating layers of Eaton-Dykman250 paper each about 0.006 in. thick to 1 layer of conventionalpolyethylene film 0.001 in. thick, the width of the membrane beingapproximately 0.375 in. The anode was platinized titanium and thecathode was graphite. Application of four volts for 9 hours resulted ingreater than 99% removal of manganese as manganese hydroxide and anequal recovery of sulfuric acid of suitable quality for reuse as aleaching agent. Supernatant water from the manganese precipitate was ofdischargeable quality.

It will thus be seen that according to the present invention a membraneassembly and electrochemical cells utilizing same have been providedwhich are inexpensive, capable of operation over long periods of timewithout fouling, efficient, have no membrane potential, and havenumerous advantages over prior art membranes and electrochemical cells.Additionally, a number of processes utilizing the membrane assembly--orlike membrane assemblies--and electrochemical cells have been providedwhich heretofore were not possible to accomplish, or were accomplishedwith much less efficiency.

While the invention has been herein shown and described in what ispresently conceived to be the most practical and preferred embodimentthereof, it will be apparent to those of ordinary skill in the art thatmany modifications may be made thereof within the scope of theinvention, which scope is to be accorded the broadest interpretation ofthe appended claims so as to accomplish all equivalent assemblies andmethods.

What is claimed is:
 1. A method of concentrating hexavalent chromiumutilizing an electrolysis system including a block of ion-transfermembrane material, means defining an electrode chamber in the blockextending the length of the height of the block, and anode disposed inthe electrode chamber in the block, and a cathode formed to allow liquidpassage therethrough surrounding the membrane material block, with acasing surrounding the cathode, the method comprising the stepsof:passing chromium contaminated water into the casing, between thecasing and the cathode; effecting circulation of the chromiumcontaminated water around the periphery of the cathode, the waterpassing through the cathode into operative association with themembrane; supplying DC current to the anode and cathode to effectconcentration of the hexavalent chromium from the chromium contaminatedwater in the chamber within the membrane; and withdrawing relativelypure water from the casing and withdrawing concentrated chromic acidfrom the chamber within the membrane block.
 2. A method as recited inclaim 1 wherein the chromium contaminated water is chromium platingrinse water.
 3. A method of concentrating hexavalent chromium, utilizingan electrolysis system including an anode chamber having an anode andanolyte, and a cathode chamber including a cathode and catholyte, and amembrane between the anode chamber and the cathode chamber, the membranecomprising an anion permeable, non-ionic, semi-permeable or waterimpermeable non-permselective membrane capable of providing passage ofanions thereacross over extended operations without destructiveswelling, clogging, chemical reaction, or consumption thereof; themethod consisting of the steps of:passing water contaminated withhexavalent chromium into the cathode chamber in steady contact with thecathode; subjecting the water contaminated with hexavalent chromium toelectrolysis to effect concentration of the hexavalent chromium in theanolyte; and withdrawing relatively purer water from the cathode chamberand concentrated chromic acid from the anode chamber.
 4. A method asrecited in claim 3 wherein the water contaminated with hexavalentchromium also includes trivalent chromium, and comprising the furtherstep of oxidizing the trivalent chromium to hexavalent chromium beforepassing the water contaminated with chromium to the cathode chamber. 5.A method as recited in claims 3 or 4 utilizing a plurality of seriesconnected cathode chambers with a common anode chamber, the anode andcathode chambers being separated by an anion permeable membrane, andwherein said method is practiced by feeding chromium contaminated waterto the bottom of the first cathode chamber; supplying current ofsufficient strength to effect electrolysis to the anode; and withdrawingwater having less than 1 ppm chromium from the top of the last cathodechamber.
 6. A method as recited in claim 5 wherein the water formed hasless than about 0.01 ppm chromium.
 7. A method as recited in claim 3comprising the further step of oxidizing the contaminants in thecontaminated water before subjecting the water to electrolysis.
 8. Amethod as recited in claim 3 practiced to achieve a Faraday efficiencyof at least about 35% with an initial hexavalent chromium concentrationof about 1000 ppm.
 9. A method as recited in claim 3 wherein the waterwithdrawn from the cathode chamber is used for chromium plating rinsing.10. A method of concentrating hexavalent chromium, utilizing anelectrolysis system including an anode chamber having an anode andanolyte, and a cathode chamber including a cathode and catholyte, and amembrane between the anode chamber and the cathode chamber, the membranecomprising an anion permeable, non-ionic, semi-permeable or waterimpermeable non-permselective membrane capable of providing passage ofanions thereacross over extended operations without destructiveswelling, clogging, chemical reaction, or consumption thereof; themethod consisting of the steps of:passing chromium plating rinse watercontaminated with hexavalent chromium into the cathode chamber in steadycontact with the cathode; subjecting the water contaminated withhexavalent chromium to electrolysis to effect concentration of thehexavalent chromium in the anolyte; and withdrawing relatively purerwater from the cathode chamber and concentrated chromic acid from theanode chamber.
 11. A method of concentrating hexavalent chromium,utilizing an electrolysis system including an anode chamber having ananode and anolyte, and a cathode chamber including a cathode andcatholyte, and a membrane between the anode chamber and the cathodechamber, the membrane comprising an anion permeable, non-ionic,semi-permeable or water impermeable non-permselective membrane capableof providing passage of anions thereacross over extended operationswithout destructive swelling, clogging, chemical reaction, orconsumption thereof; the method comprising the steps of:passing chromiumplating rinse water, contaminated with hexavalent chromium, into thecathode chamber in steady contact with the cathode; subjecting thechromium plating rinse water to electrolysis to effect concentration ofthe hexavalent chromium in the anolyte; and withdrawing relatively purewater from the cathode chamber and concentrated chromic acid from theanode chamber.
 12. A method as recited in claim 3 wherein the chromiumplating rinse water has a chromium concentration of about 209 parts permillion or less.
 13. A method as recited in claim 11 comprising thefurther step of returning the water withdrawn from the cathode chamberfor chromium plating rinsing.